1. Field of the Invention
The present invention relates to a substrate for use in a perpendicular magnetic recording hard disk and a method for producing the same.
2. Description of the Related Art
In the field of magnetic recording, information recording by hard disk apparatus is indispensable for primary external storage apparatuses in computers including personal computers. In recent years, the improvement in magnetic recording density in hard disk apparatus has been significant, rising at a rate of 100% or more per year. The recording density has reached close to 60 Gbits/inch2 at research levels and 30 Gbits/inch2 even at product levels.
This high recording density has been attained by remarkable improvement in the performance of various mechanical and electronic elements such as electronic components constituting hard disk apparatus and related software. In particular, this high recording density is largely attributed to the progress of magnetic heads (thin-film heads, MR heads, GMR heads, etc.) for reading/writing recorded information and the progress of error correction methods (software) for improving the reliability of signals having been read. Nevertheless, there are no particular changes in the basic recording system and the configuration of the apparatus, that is, the apparatus is configured on the basis of the horizontal magnetic recording system.
However, because of the improvement in magnetic recording density, the volume of a recording layer per bit for magnetic recording has decreased abruptly. In order to improve the recording density, it is necessary to improve both the linear recording density in the radial direction. However, problems are caused particularly in the improvement in the linear recording density because of reasons in the principle of the magnetic recording system. This will be described below in detail.
The magnetic recording system is broadly divided into the horizontal magnetic recording system and the perpendicular magnetic recording system as schematically shown in FIG. 2 and FIG. 3 depending on the magnetic unit (bit) arrangement system for holding information on a recording medium.
The horizontal magnetic recording system is a system for carrying out recording so that the magnetic information units formed of S-N magnetic poles become parallel with the plane of a recording medium, and this system is used for conventional hard disk media. On the other hand, the perpendicular magnetic recording system is a system for carrying out recording so that the magnetic information units become perpendicular to the plane of a recording medium, and this system is widely used for videotapes and the like requiring high-density recording.
In the case when the recording density per unit area is improved in magnetic recording, it is necessary to reduce the volume of the magnetic recording unit (bit), as a matter of course.
However, because of problems in the principle of magnetic theory, when the volume of the magnetic substance generating the effects of a ferromagnetic material for carrying out recording is decreased, it is known that stability is not always maintained as the volumes decrease. Owing to the competitiveness between thermal energy kT (k: Boltzmann constant, T: absolute temperature) at room temperature and anisotropic energy KuV (Ku: anisotropic energy, in particular, crystalline magnetic anisotropic energy in the case of magnetic recording, V: the volume of a unit recording bit) for holding the ferromagnetic substance in one direction, it is known that the volume of the magnetic recording unit is extremely small and that the magnetized state of the ferromagnetic substance becomes unstable even at room temperature when kT˜KuV is nearly established. In the case when the magnetized volume per bit is extremely small as described above, the state wherein a ferromagnetic substance becomes a paramagnetic substance is referred to as super paramagnetism. It is known that there is a limit dimension (critical volume) at the time when the ferromagnetic substance becomes super paramagnetic, although the dimension differs depending on the magnetic recording material.
In actual magnetic recording, when the recording unit volume is decreased close to the critical dimension by raising the recording density, a problem becomes manifest before the super paramagnetism is reached. In other words, a problem of deteriorating magnetically recorded information (reducing the S/N ratio of the signal read by the magnetic head) occurs, since the ferromagnetic state by magnetic recording decays with time in a relatively short time and the magnetization direction becomes random. If this phenomenon occurs in a magnetic recording, recorded information that was written becomes unable to be read after a lapse of some time or writing itself cannot be carried out. In recent years, this decaying of recording bits owing to this super paramagnetism, referred to as a “Brownian motion” problem, has become an extremely serious problem resulting in determining the limits of magnetic recording.
Although the specific numerical value of the recording limit owing to Brownian motion in the conventional horizontal magnetic recording system is not known, it is assumed to be approximately 100 Gbits/inch2 in terms of the recording density of a hard disk medium.
As systems for solving the problem of the recording limit owing to Brownian motion in the conventional horizontal magnetic recording disk medium, various new recording systems have been proposed. A system regarded and examined as the most promissing system is the perpendicular magnetic recording system. In the perpendicular magnetic recording system, the magnetic field from adjacent bits becomes the same direction as the magnetization direction, whereby the stability of the recorded and magnetized bits is supported. In other words, a closed magnetic circuit is formed between adjacent bits, whereby a self-demagnetizing field (hereafter referred to as a demagnetizing field) by self-magnetization in the perpendicular magnetic recording system is small in comparison With the horizontal magnetic recording system, and the magnetized state becomes stable. On the other hand, in the horizontal magnetic recording system, as the linear recording density is raised, adjacent recording bits become closer to each other, and the demagnetizing field becomes larger. In order to raise the linear recording density further, it is necessary to extremely reduce the thickness of the magnetic recording layer so that a rotation magnetization mode does not occur inside the magnetic recording layer. In the horizontal magnetic recording system, as the recording density rises, the volume of the recording bit decreases three-dimensionally. In the perpendicular magnetic recording system, it is not necessary to reduce the thickness of the magnetic film in accordance with the improvement in recording density. In consideration of these, in the perpendicular magnetic recording system, the demagnetizing field can be reduced and the value of KuV can be obtained securely, whereby the stability of magnetization against Brownian motion is high. Therefore, it is possible to say that the perpendicular magnetic recording system is a recording system that can extend the recording limit much further. The recording medium for the perpendicular magnetic recording system is highly compatible with the horizontal recording medium, and technologies basically similar to those used conventionally can also be used for writing and reading of magnetically recorded information.
However, in detail, there are some points causing problems in commercialization of the perpendicular magnetic recording system. One of them is the construction of a magnetic medium. FIG. 2 is a schematic sectional view showing the film construction of a horizontal magnetic recording medium, and FIG. 3 is a schematic sectional view showing the film construction of a perpendicular magnetic recording medium. In the horizontal magnetic recording medium shown in FIG. 2, a nonmagnetic under layer 103 having a thickness of 20 to 30 nm and a recording layer 104 having a thickness of 20 to 30 nm are formed on a substrate 101. In the perpendicular magnetic recording medium shown in FIG. 3, a soft magnetic layer 105 having a thickness of 100 to 500 nm and a recording layer 104 having a thickness of 20 to 30 nm are formed on a substrate 101.
As substrates for the horizontal magnetic recording system, Al—Mg alloy substrates plated with NiP are mainly used for 3.5-inch substrates, and glass substrates are mainly used for 2.5-inch substrates. On each substrate, a nonmagnetic under film (mainly made of Cr or Cr alloy), a recording film (mainly made of Co—Cr-based alloy), a protection film (mainly made of DLC: diamond-like carbon), a lubrication film, etc., are formed.
In reality, one or more buffer layers are frequently formed between the substrate and the under film or between the under film and the recording film. In a typical construction of films with respect to the thickness values thereof, the thickness of the under film is approximately up to 30 nm and the thickness of the recording film is approximately up to 20 nm at a density of approximately 20 Gbits/inch2.
On the other hand, a perpendicular magnetic recording medium comprises a soft magnetic backing layer (typically made of permalloy or the like), a recording film (candidate materials include a CoCr-based alloy, a multi-layer film obtained by alternately laminating a PtCo layer and ultra-thin films of Pd and Co to form several layers, and a SmCo amorphous film.), a protection film, a lubrication film, etc., on a substrate. There are two significant differences between the horizontal magnetic recording medium and the perpendicular magnetic recording medium, that is, the Cr-based nonmagnetic under layer for the horizontal magnetic recording medium and the soft magnetic backing layer for the perpendicular magnetic recording medium, and the compositions of the recording layer. In particular, the backing. layer of the perpendicular recording medium is required to have soft magnetism and a thickness of approximately 100 nm to 500 nm. The soft magnetic backing film is a path for magnetic flux from the upper recording film and also a path for writing magnetic flux from the recording head. Hence, the film plays the same role as the iron yoke of a permanent magnetic circuit. The film is required to be relatively very thick in comparison with the film of the horizontal recording medium as described above.
Forming the soft magnetic backing film of the perpendicular recording medium is not easy in comparison with forming the nonmagnetic Cr-based under film of the horizontal recording medium.
Usually, all the films of the horizontal recording medium are formed by a dry process (mainly magnetron sputtering). Even in the perpendicular recording medium, film formation by a dry process is a natural trend.
However, forming the soft magnetic backing layer of the perpendicular recording medium by sputtering has problems. Magnetron sputtering is a physical deposition process widely used to form not only magnetic recording media but also metallic thin films. In this process, a target is placed in an atmosphere of thin inert gas, an electrode placed near the target or the target itself is used as an electrode, and target atoms are physically driven away by gas plasma obtained by applying a high frequency wave across the electrodes thereby to form a film. In order to increase the. speed of film formation, a method wherein a permanent-magnet magnetic circuit is disposed on the rear side of the target and the magnetic force leakage to the front side is used to raise the density of plasma is generally used. However, in the case when an attempt is made to form a soft magnetic layer for perpendicular magnetic recording by this magnetron sputtering system, many problems occur. Since the target has soft magnetism, most of the magnetic flux generated from the magnetic circuit passes through the inside of the target and hardly leaks to the outside of the surface of the target. When the leakage amount of the magnetic flux is low, generated plasma becomes weak and unstable, whereby the film formation speed by sputtering cannot be sufficiently attained. In addition, the magnetic flux leakage portion of the target is preferentially subjected to sputtering. However, the leakage of the magnetic flux at the portion subjected to sputtering is larger than that at the fringe portion because of the magnetic flux having intrinsically passed through the inside of the target, and the leakage portion is increasingly subjected to sputtering and is pitted, whereby partial wear of the target occurs. In other words, when a soft magnetic target is subjected to magnetron sputtering, the sputtered portion is worn in the shape of a V-groove, and the backing plate is exposed in a relatively short time, whereby the life of the target is shortened. On the other hand, if a thin target is used in order to increase the leakage of magnetic flux on the target, the life of the target is shortened, and the target is required to be exchanged frequently. If an attempt is made to increase the thickness of the target in order to extend the life of the target, most of the magnetic flux from the magnetic circuit at the bottom passes through the inside of the target, and the external leakage of the magnetic flux becomes nearly lost. Therefore, the thickness cannot be increased significantly. Since the leakage of the magnetic field cannot be made large and since local sputtering is apt to occur, the number of sputtering vacuum baths serving as components of the sputtering apparatus is required to be increased. Otherwise, thick films cannot be formed. Furthermore, partial wear of the target affects the uniformity of the thickness of a formed film and the uniformity of the composition of an alloy. On the other hand, since the recording layer formed on the soft magnetic backing film is relatively thin, film formation is possible without problems in a dry process and any other process. Although the forming of the soft magnetic backing film for a perpendicular recording medium can be carried out in principle by the conventional sputtering method, the film formation has big problems in mass production efficiency and productivity as described above.
Moreover, as a problem peculiar to the perpendicular magnetic recording medium, noise occurs from the magnetic film of the perpendicular recording medium. The noise is broadly divided into medium noise from the recording magnetic film and spike noise from the soft magnetic backing film. The former also occurs in the case of horizontal recording. However, the latter, that is, the spike noise from the soft magnetic backing film, is peculiar to the perpendicular recording film. It has been recently thought that the spike noise occurs since the magnetic head picks up the magnetic field leaked from magnetic domain walls present in the soft magnetic backing layer. Reducing the spike noise from the soft magnetic backing film is one of the critical points required to be attained in order to commercialize the perpendicular recording film.